In a typical emulsion aggregation toner process, the toner particles' are grown in the slurry and “frozen” at the end of the emulsion aggregation phase. This is done by adjusting the toner slurry pH to a desired pH range, such as around 7-8, by adding, for example, a sodium hydroxide solution (NaOH), with or without a chelating agent, such as ethylene diamine tetraacetic acid (EDTA). Like most strong bases, the NaOH solution has a pH of about 14, if added too quickly, the NaOH solution can shock the toner slurry system when it is used to freeze the slurry. This shock affects the final dry toner properties, thus resulting in print, image, or other such unacceptable defects.
After the emulsion aggregation step, the temperature of the slurry is increased to promote toner coalescence. During this temperature ramp in the coalescence step, the pH of the slurry can become unstable. Thus, the slurry pH must be continuously and carefully monitored and adjusted with periodic amounts of NaOH to maintain the appropriate and specifically desired pH-temperature profile and to prevent further undesired particle growth or particle agglomeration. Any error in NaOH solution addition, such as changes in solution flow rate to the slurry or a pH meter reading malfunction can lead to a batch failure. These errors have increasing probability of occurrence with increasing scale as manufacturing is highly automated; thus potentially increasing the number of failed batches.
In some toner processes, such as those used for making polyester toners, there is a very narrow process latitude. Thus, the above-described problems are particularly difficult to avoid in such processes.
As a result, there exists a need to create pH-temperature stability during the temperature ramp from freezing to the coalescence step to improve process robustness, in particular during temperature ramps, and to ensure that toner particles do not grow out of control and maintain their desired specifications.